Field of the Invention
The present invention relates to the technical field of transformer on-load tap changers, and in particular, to an external combined on-load tap changer.
Related Art
The on-load tap changer achieves the purpose of changing the output voltage of the transformer by changing a winding tap connecting the transformer and changing the turn ratio of primary and secondary windings in the case that the transformer is on load (i.e., no power outage).
In the traditional technology, the combined on-load tap changer usually includes a change-over switch and a tap selector; the change-over switch is above, the tap selector is below, the middle is connected mechanically, which are entirely placed in the transform oil tank from the transformer cover flange hole. The change-over switch is generally a cylindrical member, sealed in its own separate cylindrical oil chamber, the tap selector usually is a cage structure, with terminals thereon, used for correspondingly connecting transformer coil taps, directly immersed in the insulating medium of the transformer oil tank.
Such a structure has the following disadvantages.
The transformer needs to reserve a switch position, so that the volume of the transformer increases. If the change-over switch oil chamber is leaking or damaged, it may pollute the transformer oil, resulting in larger losses and accidents.
The existing external on-load tap changer is only composite, such a composite switch combines switching and selection functions, that is, transition contacts, arcing contacts, transition resistors and main contacts are composite on a movable contact assembly, and the actions of transition and switching are performed once through rotation of the spindle of the switch. Due to structural reasons, the composite switch is often only applicable to on-load tap changers with lower voltage and small capacity, not suitable for use requirements of high voltage and large current.
Moreover, the existing combined on-load tap changer only has a Y-type connection manner, and for a large Δ connected transformer, 3 switches need to be selected, each controlling one phase therein, resulting in that the volume of the transformer increases, wiring is difficult, and the cost increases.
The external combined on-load tap changer has its unique advantages, for example, it has an individual oil chamber separated from the transformer, transportation can be separated from that of the transformer, and the like.
The switch body (core) of the external combined on-load tap changer in the prior art is directly mounted in the switch box, and the switch body is inseparable from the whole box, which brings about inconvenience to installation, repair and maintenance of the switch.